The invention relates generally to methods and structures for joining a first part onto a second part. In particular, the invention relates to an expandable bolt device for joining a fragile part to a second part.
Expandable bolt devices are known from prior art, using screws or bolts and nuts. U.S. Pat. No. 3,942,407 describes a screw anchoring device adapted to be inserted in a bored hole and of the type having a tubular anchoring member molded from plastics material and provided with longitudinally extending slits or slots. The anchoring member co-operates with a screw in such a manner that the anchoring member may be axially compressed and thereby radial expanded by tightening of the screw.
Using an expandable bolt device joining a fragile part, such as a ceramic part, onto a second part, such as a steel part, entails specific problems relating to the fragility of the ceramic part and consequent risk of breakage.
A first problem relates to the forces that act upon such fragile parts. The forces that act upon a fragile part as a consequence of joining with another part could result in exceeding material property limits of the fragile part, leading to its destruction. Particularly when the expandable bolt device is mounted in a hole in the fragile part, the radial forces exerted by the expanded bolt upon the fragile part can result in damage. This is particularly a problem when the joined parts are subjected to repeated thermal cycling. Due to differences in thermal expansion of the different materials used, the forces upon the fragile part can become excessive. To prevent destruction of the fragile part it is necessary to minimize the forces acting upon the fragile part, in other words to ensure that the forces exerted upon the fragile part never exceed the maximum allowable forces for the fragile part.
A second problem relating to differences in thermal expansion is that, when the joined parts are subjected to thermal cycling, play can develop in the joint so that the joint loosens and allows the parts to move with respect to each other, which is undesirable.
It is accordingly an object of the invention to provide an expandable bolt device that can be used to mount a fragile part securely onto a second part with minimal risk of damaging fragile part. It is a further object of the invention to provide an expandable bolt device that can be used for a vertical furnace, to mount a quartz inner doorplate to a stainless steel outer doorplate.
Embodiments of the present invention provide an expandable bolt device, co-acting with a hole in a surface of a fragile part such that the expandable bolt device exerts minimal and preferably no radial forces upon the fragile part.
According to one aspect of the invention, a surface of the fragile part is provided with a hole, the hole comprising a first section near a surface and a second section farther from the surface. The first section has a first width and the second section has a second width that is larger than the first width. An expandable bolt device is provided with an expandable section at one end. The expandable bolt fits in the hole such that the expandable section is located in the second section of the hole. In the illustrated embodiment, the expandable section is radially expandable from a width smaller than the first width to a width larger than the first width but smaller than the second width.
According to a further aspect of the invention, an expandable bolt device joins a part with a second part. The expandable bolt device is inserted with an expanable end into a hole in the fragile part. At an opposing end, the bolt device is mounted through a hole in the second part. In the illustrated embodiments, the bolt device is provided with springs that exert a gentle force on the expandable bolt device such that the fragile part is pulled against a bearing surface.
In accordance with another aspect of the invention, a method is provided for mounting a fragile part to a second part. The method includes providing a cylindrical hole in a surface of the fragile part. The hole has a first section adjacent to the surface, and a second section located further away from the surface. The first section having a first width and the second section having a second width larger than the first width. The method further includes providing an expandable bolt device with a shaft and a radially movable locking element proximate a distal end of the shaft. The distal end of the expandable bolt device is inserted into the hole. Axially moving the shaft with respect to the locking element causes outward radial movement of the locking element within the second section of the hole.